1. Field of the Invention
This invention generally relates to recycled rubber products and more particularly to composites made from ground rubber particles and polyurethane binders.
2. Description of the Related Art
Old tires and other rubber articles may be ground into rubber particles suitable for recycling. At the present time in the United States, Canada, and Mexico, over 20 million pounds of recycled rubber are used each month. Recycled rubber is used to produce many finished goods, including welcome mats, animal mats, truck-bed liner mats, playground mats, speed bumps, railroad crossing pads, carpet pads, running tracks, and bases for traffic control devices. Presently, finished goods produced from recycled rubber are made by either vulcanizing the rubber or my making composites using ground rubber and polyurethane binders.
In the vulcanizing process, the rubber is first dried to reduce the water content to less than 0.5%. The rubber is then vulcanized with sulfur. In some cases, the rubber is first treated to give a product with higher tensile strength. Rubber treated for vulcanization cannot be used in the polyurethane binder process.
In the polyurethane binder process, a mixture of ground rubber (crumb rubber) and one or more polyurethane binders is molded or formed and cured. The binder may be cured in a “hot-cure” process, at elevated temperatures, or in a “cold-cure” process, at ambient temperature. Hot-cure processes are typically performed under elevated pressures, and are typically used in manufacturing, for example, for making welcome mats, animal mats, speed bumps, and the like. Cold-cure processes are typically performed at ambient or low pressures, and are typically used when the composite is cured on-site, for example, for playground surfaces, running tracks, and floors for animal stalls. The binder in a hot-cure process typically cures in minutes, whereas the binder in a cold-cure process typically takes days to fully cure.
Many types of binders are used in the polyurethane binder process. The cost of these binders ranges from about $0.70 per pound to about $1.40 per pound. In commercial production processes, the ground rubber is mixed with from about 3% to 20% of the binder, and the mixture is molded into the finished product. Dry rubber, rubber containing less than 0.5% water, is desirable, but not necessary. A finished product made with rubber with a high water content has lower tensile strength than one made with dry rubber, however. Most commercial crumb rubber contains more than 1% water, and few manufacturers dry the rubber.